There is an increasing demand for ready to drink tea or iced tea and several methods for extracting the tea solids in concentrated form, and such that they are cold water soluble have been investigated.
Several attempts have been made to improve the cold water solubility of instant tea but they have not been fully successful. Cold water soluble instant tea was generally prepared by cooling a hot water extract of leaf tea, and separating the precipitated insoluble portions by filtration or centrifugation. The insoluble material, which is often referred to as the tea cream, is discarded and the clear extract is dried. This process entails a considerable loss in yield.
United States patent specification U.S. Pat. No. 5,827,560 (Nestec S. A.) discloses a continuous process for producing a cold water soluble tea extract. Tea solids are extracted from tea leaves using an extraction liquid to provide a tea extract that contains soluble tea solids and insoluble tannins. The insoluble tannins are separated from the tea extract by cooling the extract to below about 15.degree. C., usually to about 5.degree. C. to 10.degree. C. (i.e. chill decreaming). The insoluble tannins are then oxidised and solubilised under raised temperature and pressure conditions to provide a solubilised tannin liquor. Ground green tea can be added to the tannin liquor prior to oxidation to assist the oxidation reaction. The solubilised tannin liquor is either returned to the extraction liquid during liquid extraction of tea solids or added to the tea extract after separation of the insoluble tannins. A tea extract that contains solubilised tea tannins is collected.
United States patent specification U.S. Pat. No. 3,163,539 (Standard Brands Incorporated, 1964) discloses an improved process for preparing cold soluble tea having acceptable flavour, aroma and colour. The process described involves preparing a water extract of tea leaf and treating the cold water insoluble constituents of the extract with sodium hydroxide, preferably in the presence of oxygen. This treatment may be done either after or without separation of these constituents from the cold water soluble part of the extract. Through this treatment the previously insoluble constituents are rendered cold water soluble. The resulting alkaline liquor is neutralised and the total tea liquor is concentrated by evaporation to produce instant tea. According to this disclosure oxygen plays an important role, along with alkali, in the solubilisation reaction and introducing air or pure oxygen can increase solubilisation.
Though by the process described in U.S. Pat. No. 3,163,539, alkali induced solubilisation is enhanced by introducing air into the extract, this may lead to excessive darkening of the extract. Solubilisation should therefore be achieved rapidly so as to prevent undue darkening of the colour of the extract. Moreover introduction of air or oxygen into the strongly alkaline mixture leads to increased foaming of the extract during solubilisation with alkali.
European patent specification EP 699393 A (Nestle) discloses a process for recovering tea cream effectively by mixing the insoluble cream separated by chilling with spent black tea leaves or with green tea leaves and concentrating and drying the pooled hot water soluble and cold solubilised extracts. According to this process in order to improve the solubilisation, it is essential to separate the cream by chilling and then treat the same with spent tea leaves or green tea leaves.
It has now been found that it is possible to improve solubilisation of the tea solids without the use of alkali, acids etc. and without the process of cold decreaming. This process may be made more cost effective as it can avoid the process of chilling which involves the use of a refrigeration system. The extract thus produced has good clarity and has high levels of soluble tea solids.